Abstract :
The decays Bd→π+π− and Bs→K+K− are related to each other by interchanging all down and strange quarks, i.e. through the U-spin flavour symmetry of strong interactions. A completely general parametrization of the CP-violating observables of these modes is presented within the framework of the Standard Model, allowing the determination of the angles β and γ of the unitarity triangle. This strategy is affected neither by penguin contributions nor by any final-state-interaction effects, and its theoretical accuracy is only limited by U-spin-breaking corrections. If the B0d–B0d mixing phase 2β is determined separately, for example with the help of Bd→J/ψ KS, γ can be extracted with a reduced U-spin flavour symmetry input. A variant of this strategy to determine γ, which uses Bd→π∓K± instead of Bs→K+K− and relies – in addition to the SU(3) flavour symmetry – on a certain dynamical assumption, is also briefly discussed.
